In many modes of the related art of utilizing the geothermal energy, for example, a heat exchanging duct or pipe using air or water as a heat transfer medium is extended from a basement, an underground buried pipe or the like into a building so that the heat transfer medium warmed or cooled in the ground is circulated in the building for the air-conditioning purposes or so that a motive power is extracted by an equipment to be actuated by the heat exchanges. Alternatively, an underground constant temperature layer (or an underground portion laid in selected depth with constant temperature throughout a year) at a low temperature is utilized so that the geothermal energy is utilized by storing food or the like in a cave reaching the underground constant temperature layer, by storing goods in a hole and by covering or burying the goods in the ground, or the like.
The underground temperature change is caused within the range of a constant depth from the ground surface mainly by the irradiation of the solar heat. The ground deeper than the aforementioned constant depth is an underground constant temperature layer, in which the temperature hardly changes among the seasons, and the thermal energy rises the higher as the layer becomes the deeper. The constant depth from the ground surface, i.e., the underground constant temperature layer takes a lower temperature in summer than that of the ground surface and a higher temperature in winter than that of the ground surface. This thermal energy of the underground constant temperature layer can be utilized for the cooling purpose in summer and for the warming purpose in winter, if it is introduced into a building. And, the thermal energy in the aforementioned underground constant temperature layer is in fact an inexhaustible natural energy and is advantageous over other natural energies (e.g., a solar heat or solar light, wind power or water power) in that it is stable and usable (for introducing the thermal energy easily because it is present just under the building). The aforementioned example of utilizing the geothermal energy notes that advantage, but it cannot be said that the geothermal energy is sufficiently utilized. Therefore, means for effectively utilizing the thermal energy in the underground constant temperature layer are studied while using a supplementary device such as a heater or an air conditioner, or while using natural energies such as solar heat, solar light, wind power, or water power, in order to prevent limited fossil energies such as petroleum, gases, and coal from being exhausted.